Within the last few years, a considerable amount of attention has been directed to electromagnetic interference (EMI) from a wide variety of relatively low power devices, such as home computers, calculators, video games and similar devices. These individual devices create a certain amount of electromagnetic interference which can be quite troublesome when components thereof enter the high frequency range of 1-1,000 Megahertz. Such frequencies are reached in digital devices, such as home computers, video games and calculators, when the signal rate is drastically increased. When rapid signal pulses are employed in processing digital information and in communicating this information, substantial harmonic frequencies are created, especially when relatively square pulses are employed. Radiated and conducted EMI is thus possible by operation of such digital processing equipment. The pollution quotient is magnified by the greatly expanding number of these devices now being clustered. The basic approach to attenuation of the radiation EMI has been to encapsulate or enclose the devices in an electrically conductive shell. Metal housings were first employed for this purpose; however, for various reasons, such as appearance, ease of manufacturing and assembly and safety, digital devices have generally been converted to plastic housings or containments. Such plastic housings provide little or no shielding; therefore, substantial effort has been devoted to the use of coatings on plastic housings to shield interior circuits from radiation of EMI to the surroundings. This attempt to shield the compartment itself is quite expensive and involves metal coatings which may crack or flake. In addition, access openings and doors had to be separately sealed to complete the necessary shielding from radiation by the equipment. To overcome these shielding problems, conductive plastic materials have been developed by compounding conductive particles into the plastic. Such conductive particles such as zinc, copper, nickel, graphite and carbon black have been proposed for compounding with plastic. In addition, certain techniques are known for rendering the plastic itself partially conductive to the extent that it can possibly provide a shielding effect for high frequency radiation from the interior of a digital processing device. Such attempts to shield the device itself from EMI radiation have proven somewhat satisfactory; however, such shielding does not resolve problems created by harnesses interconnecting the device with external appliances such as keyboards and displays. After shielding the device itself, it was found that the harnesses, including a plurality of individual signal conductors or power conductors, could present a certain amount of EMI which will affect the electromagnetic compatibility (EMC) of many devices.
With the mushrooming of sales and the high concentration of personal computers, video games, and related electronic equipment, regulations are being issued to affect the EMI caused by harnesses and other external wiring for digital processing devices. This situation has presented a new round of efforts for rendering consumer products compatible with existing and proposed regulations regarding EMI. The EMI problem exists even though the device itself has circuits designed for reducing conducted and radiated EMI. Also, the problem exists even when adequate shielding is provided for the device. There is still a source of interference created by the interconnecting leads and/or connectors, such as found in harnesses.
It has become common practice ro reduce the EMI from interconnecting harnesses by using the same general concepts employed for reducing the EMI from the device itself. One of the more common approaches has been to provide a shielding sheath around the harness. This sheath must extend the total length of the harness and must be grounded at one or both ends. A shield is not only expensive, but it also provides certain technical difficulties in attempting to shield the total radiated EMI from the many conductors. This also reflects energy into adjacent conductors which can cause coupling difficulties. Coupling problems can be even more pronounced as the frequency increases and the lengths of the conductors in the harness approach approximately half wave length. Such coupling can produce cross talk which is detrimental to the efficient operation of the digital device. In addition, it is necessary to increase the thickness of the shielding layer as the frequency increases.
It has been suggested that each conductor coming into the digital device should be passed through a filter to reduce EMI at the junction of the harness with the housing. This drastically reduces conducted electromagnetic waves. By incorporating a low pass filter, the high frequencies are also dumped by connecting the filter onto a ground plane. Since a single ground plane is employed, each of the conductors passing into the digital device must be individually filtered. This requires a number of filters formed by discrete components, together with the resultant high cost.
Due to mass production requirements, various high volume digital devices, such as electronic games, video games, home computers, and calculators, include a separate structure or connector mounted on the housing of the device. This fixed connector contains a plurality of individual pins extending both into the housing and away from the housing. Internal circuits, harnesses or conductors are joined to these pins. Outside the housing, appropriate conductors or harnesses are terminated by a mass termination connector having individual contacts for each of the conductors within the harness itself. This mass termination connector is placed into the fixed connector on the housing to provide electrical connection to the fixed pins on that housing mounted connector. In this fashion, the housing mounted connector is fixed to the device and provides communication to the internal circuits, as well as communication to the outside appliances, such as displays and keyboards. With the use of these connectors on the housing, efforts have been devoted to provide filtering for each pin. This has been done by connecting each pin to a ground plane by its own capacitor. These decoupling capacitors are generally used in series with a plurality of ferrite beads mounted over individual conductors in the harness and spaced from the housing to provide a certain amount of radiation shielding. The combined beads and individual decoupling capacitors are extremely expensive and can become ineffective since the beads are susceptible to vibration and exposed to external damage.
This concept of using ferrite beads on the individual conductors before they are directed to the housing with individual decoupling capacitors at the intersection with the housing and the harness is the approach now advocated. Such structure uses discrete components and requires extensive assembly costs. Consequently, it is economically unsatisfactory even though it can be used as a part of a multipin connector mounted on the housing itself.
Another approach to solving the problem of EMI radiation and conduction by discrete components on a multipin connector is the use of separate filter pins. These filter pins are constructed from an outer layer of ferrite surrounded by a non-conducting material, such as ceramic. Around the ceramic there is provided a layer of metal. The ceramic layer creates a capacitor. By grounding the outside metal layer to a ground plate, each of the pins is coupled to the ground plate by a capacitance. The ferrite provides an inductive reactance and has a resistive component which rises rapidly to dissipate unwanted high frequencies EMI. The ferrite acts as a series resistance and inductance to concentrate and dissipate EMI. This concept of providing each pin with a separate ferrite sleeve surrounded by a ceramic sleeve and metal sleeve, for capacitor coupling to a ground plane, is extremely expensive. Each filter pin is manufactured by itself and includes its own discrete element. In addition, it is necessary to provide positive and accurate communication of the outer metal sleeve around each pin with the ground plate or plane. For that reason, the multipin connector is generally formed from metal and requires a substantial amount of manufacturing costs. When the terminals or pins of a device are multiplied, such as in video games, the cost of EMI control by individual filter pins is extremely high compared to the relatively low cost of the rest of the device.
In summary, after EMI control by design of the internal circuits and shielding of the housing surrounding the device, there is still a problem with respect to conductors being brought to the device for interconnecting the device with external appliances. When a number of individual conductors must be interconnected with the device, it is desirable to produce a single connector fixedly mounted on the housing or support wall of the device for connection between external harnesses and internal circuits. These connectors are multipin connectors secured to the device for connection with a harness on the outside and circuitboards on the inside. The outside connections still present a certain amount of EMI. Control of this EMI has been attempted by complex shielding, by the use of individual beads and decoupling capacitors for each pin of the connector and by filter pins themselves which create an inductance and capacitance for each individual pin. All of these arrangements have distinct disadvantages; however, they are being used because of the demands resulting from EMI pollution by the tremendous number of radiating devices now coming into the environment.